1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly, to a method for performing collaborative multiple-input multiple-output (MIMO) in a mobile communication system and a method for acquiring channel status information necessary for the same.
2. Discussion of the Background Art
Recently, a MIMO system has attracted attention as a wideband wireless mobile communication technology. The MIMO system serves to raise spectrum efficiency in proportion to the number of antennas, which is difficult to achieve in a communication technique employing a conventional single-input single-output (SISO) scheme.
MIMO technology refers to a multiple antenna technique for implementing high-speed communication using a plurality of antennas. The MIMO technology is classified into a spatial multiplexing scheme and a spatial diversity scheme, according to whether the same data is transmitted or different data is transmitted.
In the spatial multiplexing scheme, different data is simultaneously transmitted through multiple transmit/receive antennas. Namely, a transmitting side transmits different data using transmit antennas and a receiving side discriminates the transmitted data through interference elimination and signal processing, thereby improving transmission rate in proportion to the number of transmit antennas.
The spatial diversity scheme serves to obtain transmit diversity by transmitting the same data using multiple transmit antennas. The spatial diversity scheme is a kind of a space-time channel coding scheme. The spatial diversity scheme can maximally obtain a transmit diversity gain (performance gain) by transmitting the same data through multiple transmit antennas. The spatial diversity scheme, however, is not a method for improving the transmission rate but a method for improving the reliability of transmission by the diversity gain.
The MIMO technology may be divided into an open-loop type (e.g., Bell labs layered space-time (BLAST), space-time trellis code (STTC), etc.) and a closed-loop type (e.g., transmit adaptive array (TxAA), etc.) according to whether channel information is fed back from a receiving side to a transmitting side.
The MIMO technology may also be divided into a single-user MIMO and a multi-user MIMO according to the number of users. The single-user MIMO is performed between one mobile station having two or more antennas and a base station having two or more antennas. The multi-user MIMO is performed between two or more mobile stations each having one antenna and one base station having a plurality of antennas. The multi-user MIMO has the following advantages. First, since the mobile station needs only one transmit path, only one power amplifier is required. Further, even though one mobile station operates with another mobile station in MIMO mode, since one mobile station transmits data through one antenna, an output of the mobile station does not need to be divided according to antennas. Hence, the multi-user MIMO is not subject to a 3-dB loss generated in general MIMO. Second, a much better channel matrix can be obtained by properly selecting the two mobile stations than installing two antennas within one mobile station. According to the multi-user MIMO, since a lower correlation between user antennas is expected by simultaneously considering a plurality of users, a better type of channel characteristic matrix can be obtained.
As contrasted with the conventional MIMO applied to a single cell to achieve diversity, single-user MIMO, or multi-user MIMO, collaborative MIMO serves to raise reception performance of users in edges of a cell using a plurality of base stations in a multi-cell environment as illustrated in FIG. 1. A mobile station can increase reception performance by single-user MIMO or multi-user MIMO through multiple antennas of a base station of the same cell. A mobile station, as illustrated in FIG. 1, in an edge of a cell which is liable to be influenced by interference from neighbor cells receives a signal for the same channel from neighbor base stations, thereby implementing collaborative MIMO of diversity or spatial multiplexing (SM).
The mobile station can commonly receive data from multi-cell base stations using a collaborative MIMO system. To improve system performance, each base station can simultaneously support one or more mobile stations MS1, MS2, . . . , MSk using the same radio frequency resource. The base station can perform spatial division multiple access (SDMA) based on channel state information between the base station and the mobile station.
In collaborative MIMO, a serving base station and one or more collaborative base stations are connected to a scheduler through a backbone network. The scheduler may be operated by receiving channel state information between the respective mobile stations MS1, MS2, . . . , MSk and collaborative base stations, which is measured by respective base stations BS1, BS2, . . . , BSM through the backbone network. For example, the scheduler schedules information for a collaborative MIMO operation with respect to the serving base station and one or more collaborative base stations. Namely, the scheduler directly instructs the respective base stations to perform a collaborative MIMO operation.
Each mobile station using collaborative MIMO. reports information necessary for the collaborative MIMO operation only to the serving base station and does not report the information to the collaborative base stations. The serving base station transmits feedback information to the scheduler through the backbone network. The scheduler provides information for the collaborative MIMO operation to the collaborative base stations through the backbone network. In this case, the mobile station in an edge of a cell should transmit information about the serving base station and the collaborative base stations, demanded for the collaborative MIMO operation, together with the information for the collaborative MIMO operation, to the serving base station. Therefore, feedback overhead may be increased.